Bus bar strips, also known as laminated bus strips, are a kind of multi-layer laminated structures used as electrical couplings for power modules. They have a variety of characteristics, such as repeatable electrical properties, low inductance, high interference immunity, high reliability and space-saving, and find increasingly extensive use in power conversion devices thanks to the development of current conversion technology and increasing modularization of converters in this regard. In recent years, in particular with the increasing application of solar cells, there is great demand for developing hot-melt adhesives suited for use in bus bars of solar cells. To meet this demand, the hot-melt adhesives are required to have a light transmittance of 93%˜94%, a melting point below the laminating temperature of 140° C., good mechanical properties, high heat resistance and a thermal oxidative aging caused increase in yellowness index (ΔYI) of about 0.3.
Low-density polyethylene, LDPE for short, is one of the most commonly used polymer materials and is odorless and non-toxic and exhibits a wax like feel. It has excellent low temperature performance with its minimum usable temperature as low as −70° C.˜−100° C., high chemical stability, resistance to most acids and alkalis and good electrical insulation properties, and is hence massively used in the production of films and injection molded products. However, LDPE is very sensitive to environmental stresses and has poor mechanical properties and low thermal aging resistance. On the other hand, although polyethylene naphthalate, PNT for short, is a polyester resin superior in mechanical properties and weather resistance, its application in certain fields is limited due to a relatively high melting point.
CN101628999A discloses a method in which polyethylene (PE) scraps are melt-blended as a raw material with a pre-prepared modifier and subjected to a number of processes to produce a plastic material having the same or similar quality as new PE material. CN101121792A discloses a method in which ethylene vinyl acetate/polyethylene (EVA/PE) blends are oxidized with ozone to produce an improved foaming material. Furthermore, there are numerous literatures regarding the modification of PE by physically blending it with, for example, polypropylene (PP), polycarbonate (PC), or polybutylene terephthalate (PBT), to improve its resistance to environmental stresses and mechanical properties.
While these patent literatures provide an effective practice of performance improvement of polyethylene, none of the modified materials can meet the aforementioned requirements for a suitable hot-melt adhesive for use in solar cell bus bars. The present invention is able to prepare a polyester/polyolefin hot-melt adhesive for use in solar cell bus bars through blending an LDPE with a PNT having a relatively low molecular weight.